As for the techniques for obtaining color hard-copies, the color-recording techniques applied with an ink-jet system, an electrophotographic system or a thermal-transfer system have been studied so far. Among these systems, the thermal-transfer system has the following advantages that the operation or maintenance can readily be performed, that the apparatus can be miniaturized, that the cost can be saved and that the running cost can also be made inexpensive.
The thermal-transfer systems may be classified into the following two: namely, a system in which a transfer-sheet comprising a support bearing a thermally fusible ink-layer thereon (hereinafter referred sometimes to as a thermal-transfer recording material) is heated by a thermal-head so that the ink may be so fused as to be transferred from the thermally fusible ink-layer onto an image-receiving sheet for thermal-transfer recording use; and the other system (which may be so-called a thermal diffusion-transfer system or a sublimation-transfer system) in which a transfer-sheet comprising a support bearing an ink layer containing a thermally diffusible dye (or, a sublimatable dye) thereon is heated by a thermal-head so that the thermally diffusible dye may be transferred from the ink-layer onto the image-receiving sheet for thermal-transfer recording use. The latter system is more advantageous for full-color recording, because image gradation can be controlled by changing the dye-transfer amount so as to meet the thermal-energy variations of a thermal-head.
When a thermal-transfer recording operation is carried out in the thermal diffusion-transfer system, the dyes applicable to a thermal-transfer recording material play an important role. However, none of the conventional dyes have been satisfactory to improve the stabilities of the resulting image including, for example, the heat resistance, light fastness, fixing property and image-bleeding resistance thereof.
For improving the above-mentioned points, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP OPI Publication) Nos. 59-78893/1984, 59-109394/1984 and 60-2398/1985 disclose the image forming processes in which a chelation-type thermal-diffusible dye is used, so that an image may be formed, by the resulting chelated dye, on an image-receiving sheet for thermal-transfer recording use.
Although the above-given image-forming processes may be excellent as the processes for improving a heat resistance and a fixing property, the light fastness and image-bleeding resistance of the resulting image in storage have not been satisfactory.
As disclosed in JP OPI Publication No. 59-158287/1984, 63-74686/1988, 63-145089/1988, 59-196292/1984, 62-229594/1987, 63-122596/1988, 61-283595/1986, 1-171887/1989 and 1-204788/1989, the proposals have been made therein for adding the UV absorbent and light stabilizer into the image-receiving layer with the purpose of improving the light fastness of an image. However, when making use of the compounds described therein, the compounds may be colored or the light fastness of the resulting image and the heat stability of the compounds are not satisfactory.
Among the cases of the above-mentioned proposals, in the case of using a light stabilizers of the metal complex salt type as described in JP OPI Publication Nos. 63-122596/1988 and 1-171887/1989 and in the case of the image forming processes in which an image is formed by the chelated dyes as described in JP OPI Publication Nos. 59-78893/1984, 59-109394/1984 and 60-2398/1985, a coloration is produced when trying the combination use of an ordinary organic UV absorbent, because the UV absorbent and the metal ions are chelated together.
JP OPI Publication Nos. 63-193884/1988, 63-194981/1988, 1-208293/1989, 1-280529/1989, 1-283191/1989, 2-139551/1990, 2-252585/1990 and 2-265793/1990 and Japanese Patent Examined Publication (hereinafter referred to as JP Examined Publication) No. 3-9878/1991 propose each that an image preservability is to be improved by covering an image-recorded image-receiving layer surface with a covering film or a resin layer. In these proposals, however, there still remain the following difficulties. An image is bled when covering with a protective material; the light fastness of an image covered with a protective material and the bleeding resistance of a stored image are not satisfactory; and an image quality may not be maintained in forming the image. Even in the other cases, a coloration is similarly produced when making metal ions present as described before.